I'm not a doctor, at all. but i would lean more toward the absence of a period meaning pregnancy. I would think that you are no pregnant, especially if you had two full-blown periods. January is a long month, could it be that your periods are coming a little earlier? If you're on birth control, could that have anything to do with it? Of course, I would contact your ob/gyn if you were really concerned. Some people believe that women's periods sync up when they hang around each other. I kind of have seen this as true.

Menstrual cycle in a woman is due to ovulation which occurs once in a month. If the ovum is not fertilized, it causes the menstrual bleeding and if it is fertilized, there is no bleeding and pregnancy starts. So 2 cycles in a month means there are two ovulations in a single month; it is usually very rare. But you need not worry immediately. But if it occurs in the next month also, you must consult a gynaecologist. Your statement that your cycle comes on 12th every month shows that your cycle was so far absolutely regular because every woman faces some date problems. Do you use to take contraceptive pills? Last but not the least, I think you have already come to realise that it does not mean that you have conceived. The stop of menstrual cycle during pregnancy or due to any other cause is known as amenorrhoea.